Multicellular eukaryotic organisms are capable of responding to extracellular signals to regulate or accentuate gene expression through the process of signal transduction. A large number of the components of the signal transduction pathway are comprised of proteins that are covalently modified by ligands such as hormones, growth factors, cytokines or second messengers like camp, Ca2+, diacylglycerol, inositol-3-phosphate and others. This results in the induction of a cascade leading to gene expression in which activation of kinases that phosphorylate tyrosine or serine/threonine residues play central roles. Prominent targets of second messengers are the camp-dependent protein kinase A (PKA) and the Ca2+/phospholipid-activated protein kinase C (PKC). The protein kinase C cascade includes several enzymes that are sequentially phosphorylated in response to external stimuli. PKC established its role in signal transduction when it was demonstrated that diacylglycerol, a product of hormone stimulated phosphatidylinositol hydrolysis, was an activator of the enzyme. Multiple lipid pathways contribute to the production of diacylglycerol, the second messenger for PKC. Phorbol esters, which are potent tumor promoters, activate PKC and this discovery led to further excitement in the field (see for example, Nishizuka, 1995, pp.484-496; Nishizuka, 1992, pp.607-614).
Protein kinase C modulates diverse cellular functions such as cell cycle progression and differentiation, apoptosis, and tumor promotion (Nishizuka, 1986, pp.305-312). PKC regulates gene expression via transcription through the activation of cis-elements. It is a mediator of immune responses, hormone secretion and receptor desensitization. Many of these roles are mediated by accentuating membrane structure events. PKC phosphorylates serine or threonine residues at basic sequences with the motif xRxxS/TxRx. Thus histones H1 and IIIS, and protamine, are efficient substrates for PKC. In addition to catalyzing phosphorylation reactions, PKC autophosphorylates in vitro, (on residues Thr-641 and Ser-660 in PKCβII) by an intramolecular mechanism and this may play a role in its proteolytic activation and degradation.
Together with the fact that activation of PKC isozymes is associated with altered gene expression and its presence in the nucleus, it was expected that PKC could phosphoryate some transcription factors and thereby regulate gene expression. PKC phosphorylates CREB, the camp-responsive element-binding protein and as a result CRE binding is enhanced. NF-κB, in the cytosol of resting cells, is in a complex with I-κB. On PKC activation, by phorbol esters, the inhibitory complex dissociates and NF-κB translocates into the nucleus as an active transcription factor. C-jun, a phorbol ester-responsive transcription factor, is also regulated by PKC. Vitamin D3 receptor, a member of the steroid/thyroid hormone family, binds to vitamin D3 and associates with its DNA element thereby altering gene transcription. PKCβ phosphorylates the vitamin D3 receptor, and inhibits transcriptional activation by vitamin D3. TLS/FUS, a DNA-binding nuclear protein, is a regulator of BCR/ABL-mediated leukemogenesis (Perrotti et al., 1998, pp.4442-4455). PKCβII phosphorylates TLS/FUS and regulates its DNA-binding activity in the nucleus.
Historically, Nishizuka et al. discovered protein kinase C in rat brain as the proenzyme for a histone protein kinase (see, for example Takai et al., 1977, pp.7603-7609). This protein kinase M (PKM) was generated as a result of partial proteolysis by trypsin or a calcium-dependent protease. From biochemical studies, it was soon established that the proenzyme, PKC, exhibited kinase activity upon membrane association. It is now well established that protein kinase C, a family of serine/threonine kinase isozymes, translocates from the cytosol to another intracellular compartment like the membrane or nucleus in response to stimuli from neurotransmitters, hormones and growth factors. Most PKCs are activated by phosphatidylserine, an acidic lipid located on the cytoplasmic side of the membrane, diacylglycerol and phorbol esters. Some PKC family members also require Ca2+ and other lipid second messengers for optimal activity.